Precipitation is when an insoluble solid is separated from the soil solution due to a chemical change. There are three types of precipitation that can take place in the soil system. These are 1) precipitation of an element due to supersaturation of that element in the soil solution, 2) surface precipitation, and 3) co-precipitation of elements. These chemical reactions can potentially control the mobility and toxicity of trace metals in soil systems.
Precipitation, of an element due to its high concentration can take place in soil
solutions. This reaction takes place when the solution becomes supersaturated with respect
to that element. A supersaturated solution is a solution containing more solute (in this
case the dissolved element) than allowed at equilibrium. This type of solution is unstable
and any further addition of solute will cause the precipitation of an insoluble solid.
Solubility product, Ksp, can be used to determine whether an element will precipitate out of solution. For example we will examine the precipitation reaction of lead carbonate, PbCO3:
Pb2+(aq) + CO(aq) = PbCO3(s)
In order for the lead carbonate to precipitate out of solution, the molar
concentrations of the Pb2+ and CO32- must multiply to be
a number larger than Ksp which in this case is 1x 10-13. It is cautioned that
solubility products should not be used exclusively to determine trace element mobility and
toxicity. This is because soils are heterogeneous and other reaction and ions in the soil
solution may cause competing reactions.
Figure 1: Precipitation of a Supersaturated Solution
Source:Masterton and Hurley
Precipitation from soil solution is common for abundant elements in soil such as Al, Fe, Si, Mn, Ca, and Mg. Trace elements are present in the soil solution in much lower concentrations than needed for precipitation to occur. In rare cases, however, trace elements may be present in large quantities due to pollution. When this occurs, precipitation can control the mobility and toxicity of that element in the soil because the solid is often immobile and far less toxic to organisms.
Surface precipitation, is the three dimensional growth of a solid on the surface of
a soil particle. This process is a continuation of surface complexation and occurs when
the soil particle becomes increasingly covered by solid precipitate from the soil
solution. This process takes place in the soil system more commonly than homogeneous
nucleation. Often trace elements, such as Pb, Cr, As, and others, will precipitate on a
soil particle and become immobilized and less toxic.
Figure 2: Surface Precipitation of Heavy Metals.
Often the solubility of an element is lower in the soil solution than predicted by the solubility product because the solubility of an ion is lower in mixed ionic solution than in pure ionic solutions. One possible reason for the reduced solubility is due to co-precipitation. Co-precipitation is the incorporation of trace element into mineral structure during solid solution formation and recrystallization of minerals. This process will reduce the mobility and toxicity of the toxic trace elements that are incorporated into the mineral.
Minerals will only incorporate elements into their structure that have similar ionic radii as the elements composing the mineral. For example, during the formation of calcite, Mn2+, Cd2+, and Fe2+ can possibly be incorporated into the mineral structure. In the formation of Fe and Al-oxides, Cr3+, Mn3+, and V3+ may be incorporated into the structure. Mn and Fe-oxides have more possibility for co-precipitation than Al-oxides and aluminosilicate minerals. Co-precipitation reactions are also controlled by the rate of soil mineral dissolution.
Precipitation reactions in soils have the ability to reduce the toxicity and mobility of harmful trace elements that may be present in the soil system. These reactions occur naturally in the soil. Surface precipitation has been used as a method of natural attenuation in marshland near San Francisco. Buried paint chips containing zinc were found in the soil at the site. It was determined that the zinc that was leaching was precipitating out of soil solution into an insoluble and immobile form. The company was permitted to put a soil cap over the contaminated area, saving them the cost of excavation and incineration. Currently, precipitation is not commonly used as a method of attenuating polluted soil. This is because there are many factors that may cause the precipitates to dissolve and again become mobile. Also, other chemical reactions are competing with precipitation in the soil and may cause the pollutant to remain soluble.